Page:A Treatise on Geology, volume 1.djvu/224

 208 to the Thuringerwald, and all around the Harz. Salt occurs in the keuper and muschelkalk especially. Considerable tracts of new red sandstone adjoin the Riesengebirge; and a line of these rocks, occasionally saliferous, borders the primary ranges of the Alps, from the vale of the Danube, by Rottenmann, Radstadt, to near Innspruck. On the south side of the Alps, the range is equally extensive, from Cilli, near the Save, by Villach to St. Lorenzen on the Eisach.

Physical Geography.—Spread over so immense a space in England, the triassic system offers the remarkable fact of never rising to elevations much above 800 feet (Barr Beacon, in Staffordshire, is a gravel hill on a base of red rocks); a circumstance probably not explicable by the mere wasting of these soft rocks by floods of water, but due to some law of physical geology yet unexplained. We only can conjecture that it is connected with the repose of subterranean forces, which prevailed after the violent commotions of the coal strata, over nearly all Europe till the tertiary epoch. The red sandstone system, folding its level surfaces round the broken coal strata, seems to be like the large uplifted bed of a shallow sea, full of rocky islands, and bounded by bold promontories. (The magnesian limestone range in the north of England constitutes a fine natural terrace of 100 to 500 or 600 feet in height above the sea; its escarpment being always to the west.)

Igneous Rocks.—Almost the only cases known in England, are dykes of greenstone. One of these, the great Cockfield dyke, extends from Middleton, in Teesdale, to near Robin Hood's Bay, and passes through mountain limestone, coal, magnesian limestone, red sandstone, red marl, the lias, and oolites. Another passes from the Breiddin hills across the plain of Shrewsbury, and dislocates and alters red sandstone at Acton Reynolds. In the Isle of Arran, dykes of pitchstone, claystone, trap porphyry, &c., divide red and white sandstones, supposed to be of this era.